Method of production of a semiconductor substrate

ABSTRACT

A method of forming a Si film by a bias sputtering process comprises the steps of generating plasma between a target electrode holding a target material provided in a vacuum container and a substrate electrode holding a deposited film forming substrate, provided opposingly to the target electrode, by the use of a high-frequency energy to cause the target material to undergo sputtering, and applying a bias voltage to at least one of the target electrode and the substrate electrode to form a Si film comprised of atoms deposited by sputtering on the substrate, wherein; 
     a mixed-gas environment comprising a mixture of an inert gas and a hydrogen gas is formed in the vacuum container, and the target material is subjected to sputtering while controlling H 2  O gas, CO gas and CO 2  gas in the mixed-gas environment to have a partial pressure of 1.0×10 -8  Torr or less each, to form an epitaxial film on the substrate while maintaining a substrate temperature in the range of from 400° C. to 700° C. 
     A semiconductor substrate comprises an Si layer having a carbon content, a hydrogen content and a rare gas (X) content of C≦1×10 18  cm -3 , 1×10 15  cm -3  ≦H≦1×10 20  cm -3  and 1×10 16  cm -3  ≦X, respectively, and having a difference of 15 nm or less between a maximum value and a minimum value of surface roughness.

This application is a continuation of application Ser. No. 08/455,845filed May 31, 1995, now abandoned, which is a continuation-in-part ofapplication Ser. No. 08/160,227 filed Dec. 2, 1993, which is nowabandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method for producing a semiconductorsubstrate that can be applied in semiconductor devices or liquid-crystaldisplay devices, and more particularly relates to a method fordepositing Si film by sputtering. It also relates to a semiconductorsubstrate, and a semiconductor device or liquid-crystal display devicehaving applied it.

2. Related Background Art

Semiconductor devices used in integrated circuits are commonly comprisedof a thin-film multi-layered product formed by depositing a number ofthin films layer by layer, and the quality of each thin film and thestate of interfaces between thin films have a great influence on theperformance of devices. Hence, high-level thin-film forming andmultilayer forming techniques are indispensable for providinghigh-performance semiconductor devices. In particular, what is calledthe epitaxial growth technique, which forms an additional high-qualitycrystal on a crystal face, is a thin-film forming techniqueindispensable for existing semiconductor techniques, and much researchand development is being done on these techniques since they greatlyinfluencing device performance. A thin film obtained by epitaxial growthis called an epitaxial film.

In conventional epitaxial growth techniques, CVD processes have beenprevalent. However, film formation by CVD is, in general, carried out bya high-temperature process. For example, a film formation process for asilicon film (Si film) is carried out at a high temperature of 1,000° C.or above. This has brought about problems such as limitations onprocesses which can be used and a high production cost, ascribable tothe high temperature. Moreover, it has now become difficult to meetdemand for making dopant profiles shallower or sharper, which is arecent demand attributable to devices having been more highly integratedand having been made to have a higher performance.

Accordingly, as processes that can meet such demands, low-temperatureepitaxial growth processes have been recently reported, including MBE(molecular beam epitaxy; A. Ishikawa and Y. Shiraishi, J. Electrochem.Soc., 133, 666, 1986), as well as FOCVD that carries out film formationby mixing a gaseous material and a halogen type oxidizing agent to causechemical reaction so that a precursor serving as a feeding source of afilm forming material is formed (U.S. Pat. No. 4,800,173), HRCVD thatcarries out film formation by separately introducing into a film formingspace, gaseous materials activated in different activation spaces (U.S.Pat. No. 4,835,005), PIVD (partly ionized vapor deposition) thatutilizes an ion beam process (T. Itoh, T. Nakamura, M. Muromachi and T.Sugiyama, Jpn. J. Appl. Phys. 16553, 1977), IBE (ion beam epitaxy; P. C.Zalm and J. Beckers, Appl. Phys. Lett. 41, 167, 1982), and ICBD (ioncluster beam deposition; I. Yamada, F. W. Saris, T. Takagi, K.Matsubara, H. Takaoka and S. Ishiyama, Jpn. J. Appl. Phys. 19, L181,1980).

However, MBE requires a 800° C. or higher temperature process to form ahigh-quality epitaxial thin film and high-density doping is difficult.FOCVD and HRCVD, which utilize chemical reactions, have the problem thatby-products may be formed and be incorporated as impurities into films.In what is called the ion beam process such as IBE or ICBD, the ionsused have so much energy that substrates may be damaged, and nohigh-quality thin films usable in semiconductor devices have beenobtained under existing circumstances.

The present inventors have reported Si epitaxial growth carried out byRF-DC combined bias sputtering, employing a method in which the surfacelayer is activated while controlling any damage on the substrate byprecise control of ion energy (T. Ohmi, T. Ichikawa, et al., J. Appl.Phys. Vol.66, p.4756, 1989).

Hitherto, sputtering has not been considered utilizable to formepitaxial films because of difficulty in the controlling the damage ofsubstrates due to ion energy. It, however, has many other advantages,i.e.;

(1) it can readily form a large-area film;

(2) it can achieve a relatively simple device construction;

(3) it can match usual semiconductor processes; and

(4) it allows easy control of reaction systems.

The foregoing RF-DC combined bias sputtering can keep the advantages ofsputtering in the epitaxial film forming techniques.

The RF-DC combined bias sputtering having such advantages, however, hasthe following problems.

(1) When processing gas, argon atoms are incorporated into an epitaxialfilm in a concentration of 8×10¹⁸ cm⁻³ or more, the resulting film mayhave poor quality, and the quality may become extremely poor in a devicefabrication process carried out at temperatures higher than filmformation temperatures.

(2) The carbon can not be completely prevented from being included infilms, and the same problem as in paragraph (1) may arise when carbon isincluded in a concentration of 1×10¹⁸ cm⁻³ or more.

(3) The process has a poor step coverage.

(4) An epitaxial film formed on the (111) crystal face of an Sisubstrate or a heteroepitaxial film such as an SiGe film formed on an Sisubstrate may have defects therein chiefly caused by many filmdeposition defects, to make the state of their interface unsatisfactory.

Thus, according to such epitaxial growth, it is very difficult to grow amonocrystal on an amorphous substrate or to grow a crystal having alattice constant and a coefficient of thermal expansion that aredifferent from those of the substrate. This has imposed restrictions onsubstrate materials usable and the types of films grown. Meanwhile, inresearch and development in recent years, development is energeticallybeing made on three-dimensional integrated circuits that aim at higherintegration and higher multifunction by forming semiconductor devices ona substrate layer by layer, and also on solar cells, switchingtransistors of liquid-crystal picture elements comprised of devicesarranged in array arrays, etc., which are formed by depositingsemiconductor materials on inexpensive glass substrates. Accordingly, ithas become important to provide techniques by which high-qualitysemiconductor thin-film layers can be formed on an amorphous substratehaving a structure that is common to these devices. In recent years, TFTthin-film forming techniques for achieving such a structure areremarkably improved. For example, ion implantation is carried out on apolycrystalline semiconductor thin film or amorphous semiconductor thinfilm on an amorphous substrate to make the film completely amorphous,followed by heat treatment to obtain a polycrystalline semiconductorthin film having a grain diameter as large as several μm (T. Noguchi etal., J. Electrochem. Soc. Vol. 134, No. 7, p.1771, 1987), or anamorphous semiconductor thin film deposited on an amorphous substrate bysputtering is exposed to laser light to obtain a polycrystallinesemiconductor thin film having a grain diameter of about 400 Å (1989Spring Season Applied Physics Society, 3P-ZH-15, 16). In both examples,MOS devices having fairly good electrical characteristics and showing ahigh mobility are fabricated.

Such an ion implantation method and a laser melting method, however,both essentially require a high-temperature process, and hence they notonly are difficult to apply to three-dimensional integrated circuits,but also hinder the large-area uniform thin-film formation,low-temperature process and simple process that are recently requiredfor the purpose of adaptation to large-area substrates and costreduction. Accordingly, it is sought to provide a high-quality thin-filmforming technique by which high-quality devices can be formed onamorphous substrates by a simple low-temperature process that employsneither ion implantation nor laser melting. Meanwhile sputtering andglow discharge, are processes for forming semiconductor thin films onlow-temperature large-area amorphous substrates without use of the ionimplantation or laser melting. Under existing circumstances, however, inview of electrical characteristics such as mobility, no satisfactorydevice characteristics can be obtained in the semiconductor thin filmsdeposited by these processes, compared with the above processes.

SUMMARY OF THE INVENTION

The present inventors made extensive studies on the cause of the aboveproblems to have reached the following findings.

(1) Argon atoms deposit on a substrate at a large coefficient when thesubstrate has a low-temperature of about 300° C.

(2) The energy possessed by atoms such as Si that undergo epitaxialgrowth is so small that their surface migration may become insufficient.

(3) Carbon atoms are in the state they tend to be attached to theactivated substrate surface.

The present invention was made from such findings.

An object of the present invention is to improve a method for forming aSi-deposited film, carried out by the above RF-DC combined biassputtering, and provide a method for forming an Si-deposited film, thatcan achieve a good step coverage, may cause less film deposition defectsto give a good interfacial state.

Another object of the present invention is to provide a method forforming an Si-deposited film, that can provide high-quality crystallinethin films having a small content of argon atoms, carbon atoms and soforth that may impair the quality of thin films, contained in processinggas.

The present invention provides a method for forming an Si-deposited filmby a bias sputtering process comprising the steps of:

generating plasma between a target electrode holding a target materialprovided in a vacuum container and a substrate electrode holding asubstrate for forming thereon the Si-deposited film, provided opposinglyto the target electrode, by the use of a high-frequency energy to causethe target material to undergo sputtering; and

applying a bias voltage to at least one of the target electrode and thesubstrate electrode to form the Si-deposited film, comprised of atomsdeposited by sputtering on the substrate, wherein;

a mixed-gas environment comprising a mixture of an inert gas and ahydrogen gas which is in a content of above 10% to below 50% of theinert gas is formed in the vacuum container, and the target material issubjected to sputtering while reducing H₂ O gas, CO gas and CO₂ gas inthe mixed-gas environment to have a partial pressure of 1.0×10⁻⁸ Torr orless each, to form the Si-film on the substrate while maintaining asubstrate temperature in the range of from 400° C. to 700° C.

In the present invention, H₂ gas/(Ar+H₂) gas ratio is preferably morethan 10% to less than 50%, and more preferably more than 20% to lessthan 30%.

The present invention also embraces a semiconductor substrate, and asemiconductor device and a liquid-crystal display device that haveapplied it. More specifically, the semiconductor substrate of thepresent invention comprises a Si layer having a carbon content, ahydrogen content and a rare gas content of C≦1×10¹⁸ cm⁻³, 1×10¹⁵ cm⁻³≦H≦1×10²⁰ cm⁻³ and 1×10¹⁶ cm⁻³ ≦rare gas, respectively, and having adifference of 15 nm or less between a maximum value and a minimum valueof surface roughness. The semiconductor device of the present inventioncomprises a Si semiconductor layer having a carbon content, a hydrogencontent and a rare gas content C≦1×10¹⁸ cm⁻³, 1×10¹⁵ cm⁻³ ≦H≦1×10²⁰ cm⁻³and 1×10¹⁶ cm⁻³ ≦rare gas, respectively, and having a difference of 15nm or less between a maximum value and a minimum value of surfaceroughness. The liquid-crystal display device of the present invention isan active matrix type liquid-crystal display device comprising a pictureelement whose switching transistor comprises a Si layer having a carboncontent, a hydrogen content and a rare gas content C≦1×10¹⁸ cm⁻³, 1×10¹⁵cm⁻³ ≦H≦1×10²⁰ cm⁻³ and 1×10¹⁶ cm⁻³ ≦rare gas, respectively, and havinga difference of 15 nm or less between a maximum value and a minimumvalue of surface roughness.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic illustration of a bifrequency excitation typebias sputtering apparatus employed for the working of the method of thepresent invention.

FIGS. 2A to 2C are flow charts to illustrate a process for producing asubstrate when the method of the present invention is carried out.

FIG. 3 is a graph to show the relationship between plasma potential anda hydrogen gas content in an embodiment of the present invention.

FIG. 4 is a graph to show the relationship between carbon atomconcentration in an epitaxial film and hydrogen gas content in aprocessing gas, in an embodiment of the present invention.

FIG. 5 is a graph to show the relationship between reverse-currentdensity at p-n junctions and hydrogen gas content in an embodiment ofthe present invention.

FIG. 6 illustrates an example of the constitution of a base region in aBPT (bipolar transistor).

FIG. 7 illustrates an example of the constitution of a emitter region inthe BPT.

FIG. 8 is a graph to show the dependence of film formation rate ontarget bias in the bias sputtering.

FIG. 9 illustrates an example of a liquid-crystal display.

FIGS. 10A and 10B illustrate an example of time chart of a drivingcircuit of liquid-crystal display.

FIG. 11 is a schematic cross-sectional view of Example 8 of the presentinvention.

FIGS. 12A and 12B illustrate manufacturing steps of Example 8.

FIGS. 13A, 13B and 13C illustrate manufacturing steps of Example 8.

FIGS. 14A, 14B and 14C illustrate manufacturing steps of Example 8.

FIGS. 15A and 15B illustrate manufacturing steps of Example 8.

FIGS. 16A and 16B illustrate manufacturing steps of Example 8.

FIGS. 17A, 17B and 17C illustrate manufacturing steps of Example 8.

FIGS. 18A and 18B illustrate manufacturing steps of Example 8.

FIG. 19 illustrates manufacturing step of Example 8.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The method for forming a Si-deposited film of the present invention ischaracterized by a method of forming the Si-deposited film by a biassputtering process comprising the steps of generating plasma between atarget electrode holding a target material provided in a vacuumcontainer and a substrate electrode holding a substrate for formingthereon the Si-deposited film, provided opposingly to the targetelectrode, by the use of a high-frequency energy to cause the targetmaterial to undergo sputtering, and applying a bias voltage to at leastone of the target electrode and the substrate electrode to form theSi-deposited film, comprised of atoms deposited by sputtering on thesubstrate. A mixed-gas environment comprising a mixture of an inert gasand a hydrogen gas is formed in the vacuum container, and the targetmaterial is subjected to sputtering while controlling H₂ O gas, CO gasand CO₂ gas in the mixed-gas environment to have a partial pressure of1.0×10⁻⁸ Torr or less each, to form the Si-deposited film on thesubstrate while maintaining a substrate temperature in the range of from400° C. to 700° C.

The present invention may also embrace an embodiment in which thesubstrate is irradiated with plasma ions before the film formation toclean the surface of the substrate.

The present invention may also embrace an embodiment in which ahigh-frequency power applied to the substrate has a frequency higherthan the frequency of a high-frequency power applied to the target.

According to the present invention, H₂ O gas, CO gas and CO₂ gas in avacuum chamber of a bias sputtering apparatus are each made to have apartial pressure of 1.0×10⁻⁸ Torr or less, the substrate temperature ismaintained at 400° C. to 700° C., and the epitaxial growth by sputteringis carried out in a mixed-gas environment comprised of an inert gas anda hydrogen gas. Thus, the following advantages can be expected:

(1) The coefficient at which inert atoms, such as argon atoms and heliumatoms that are the atoms constituting the plasma, and not the atomsconstituting the deposited film, deposit on the substrate greatlydecreases. As for hydrogen atoms that are the atoms constituting theplasma and not the atoms constituting the deposited film, their force tocombine with the Si atoms constituting the substrate greatly decreaseswhen the substrate temperature is 400° C. or above. Hence, any inclusionof inert atoms and hydrogen atoms, other than deposited film constituentatoms, into the deposited film can be greatly decreased.

(2) The quantity of the carbon atoms having a great influence on thedevice performance even when incorporated into the deposited film in asmall quantity can be greatly decreased on account of the reaction withhydrogen gas.

In the present invention, the semiconductor substrate has an Si layerhaving a carbon content, a hydrogen content and a rare gas content ofC≦1×10¹⁸ cm⁻³, 1×10¹⁵ cm⁻³ ≦H≦1×10²⁰ cm⁻³ and 1×10¹⁶ cm⁻³ ≦rare gas,respectively, and has a difference of 15 nm or less between a maximumvalue and a minimum value of surface roughness. The Si layer of thepresent invention may be monocrystalline, polycrystalline or amorphous.The reason why it is preferable for the carbon content, hydrogen contentand rare gas content to be in the above amounts will be explained below.

First, with regard to the carbon content, the incorporation of carboninto the Si layer may cause a distortion of crystals and also anincrease in dangling bonds, resulting in a deterioration ofcrystallinity, and electrical characteristics after all. In particular,it may result in deterioration of characteristics concerned withcarriers, such as off-leak and mobility. It is better for the Si layerto incorporate less carbon, and the Si layer formed by sputteringaccording to the method of the present invention, conjointly with otherconditions, has made it possible for the first time to provide the Silayer containing carbon in a small amount.

Next, with regard to the hydrogen, hydrogen is known to terminatedangling bonds in polycrystalline Si films rather than inmonocrystalline Si films, and hence improves electrical characteristics.In conventional processes, Si films are treated with hydrogen byplasma-assisted CVD or the like to incorporate hydrogen. In the presentinvention, however, the hydrogen can be incorporated at the initial Sifilm formation, and hence the step of hydrogen treatment can be omitted.Moreover, since it is incorporated at the time of Si deposition, thedangling bonds can be terminated efficiently, so that the off-leakcharacteristics and S-coefficient characteristics (rise characteristics)themselves are improved. There is another advantage in incorporating ofhydrogen at the time of Si deposition since it enables removal of thecarbon. The hydrogen content is required to increase as thecrystallinity of the Si layer decreases, and the hydrogen contentincreases in the order of monocrystalline, polycrystalline andamorphous.

As for the rare gas, a sputtering gas which is a rare gas such as Ar iselectrically inert and does not deteriorate electrical characteristics.However, its inclusion in excess causes an increase in crystal strain,so that dangling bonds increase to deteriorate electricalcharacteristics. However, its incorporation in an appropriate amountcauses no deterioration of electrical characteristics themselves andenables formation of dense films, which can be oriented to (111) facewith ease. In particular, when the Si layer is monocrystalline, ashaving dense crystal structure, a denser layer can be obtained using atrace amount of rare gas (X). The monocrystalline layer, however, maydeteriorate if there is too much X. When the Si layer ispolycrystalline, Si atoms are not so densely arranged as themonocrystalline layer and Si atoms have a greater degree of freedom.Hence, the incorporation of X in a larger amount than that in themonocrystalline Si layer can be more effective. When the Si layer isamorphous, Si atoms have a much greater degree of freedom, and hence theincorporation of X in a much larger amount than that in thepolycrystalline layer enables formation of a denser film. Among raregases (X), Ar is most effective, as having an atomic radius (elementaryradius) which is close to those of Si atoms. Needless to say, other raregases (X) are also similarly effective.

Finally with regard to the surface roughness, if the surface roughnessis more than 15 nm as a pV value, electric field concentration tends tooccur because of such configuration, and causes deterioration ofbreakdown strength of gate oxide films when MOS transistors arefabricated. Since the interface between the substrate and the gate oxidefilm is very rough, problems may occur such that the mobilitydeteriorates.

Si films satisfying all of these features show very good characteristicsin respect of electrical characteristics. The carbon (C) content,hydrogen (H) content and rare gas (X) content that can bring about suchgood characteristics are C≦1×10¹⁸ cm⁻³, 1×10¹⁵ cm⁻³ ≦H≦1×10²⁰ cm⁻³ and1×10¹⁶ cm⁻³ ≦X, respectively. When the Si layer is monocrystalline, theyare more preferably C≦5×10¹⁷ cm⁻³, 1×10¹⁶ cm⁻³ ≦H≦1×10¹⁹ cm⁻³ and 1×10¹⁶cm⁻³ ≦X≦1×10¹⁸ cm³. In such a monocrystalline Si layer, they are mostpreferably C≦1×10¹⁷ cm⁻³, 1×10¹⁶ cm⁻³ ≦H≦1×10¹⁷ cm⁻³ and 1×10¹⁹ cm⁻³,respectively. When the Si layer is polycrystalline, they are morepreferably C≦5×10¹⁷ cm⁻³, 1×10¹⁷ cm⁻³ ≦H≦H×1×10¹⁹ cm⁻³ and 1×10¹⁶ cm⁻³≦X≦1×10¹⁹ cm⁻³, respectively. In such a polycrystalline Si layer, theyare most preferably C≦1×10¹⁷ cm⁻³, 1×10¹⁸ cm⁻³ ≦H≦1×10¹⁹ cm⁻³ and 1×10¹⁷cm⁻³ ≦X≦1×10¹⁹ cm⁻³, respectively. When the Si layer is amorphous, theyare more preferably C≦5×10¹⁷ cm⁻³, 1×10¹⁷ cm⁻³ ≦H≦1×10²⁰ cm⁻³ and 1×10¹⁷cm⁻³ ≦X≦1×10²⁰ cm⁻³, respectively. In such an amorphous Si layer, theyare most preferably C≦1×10¹⁷ cm⁻³, 1×10¹⁹ cm⁻³ ≦H≦1×10²⁰ cm⁻³ and 1×10¹⁸cm⁻³ ≦X≦1×10¹⁹ cm⁻³, respectively.

FIG. 1 illustrates a bifrequency excitation type bias sputteringapparatus employed for the working of the method of the presentinvention. In the drawing, reference numeral 11 denotes a vacuumchamber; 12, a target; 13, a target supporting member; 14, a substratesupporting member; 15, a permanent magnet; 16, a substrate; 17, a 100MHz high-frequency power source; 18, a 190 MHz high-frequency powersource; 19 and 20, matching circuits; 21, a low-pass filter for thetarget; 22, a low-pass filter for the substrate; 23 and 24, band passfilters; 25, a magnetic levitation type tandem turbo molecular pump; 26,a dry pump; 27, a DC power source for determining the DC potential ofthe target; 28 a DC power source for determining the DC potential of thesubstrate; and 29, a xenon lamp for heating the substrate.

The vacuum chamber 11 is made of a material that lessens gas escape. Forexample, SUS316 is used as a structural material, and the inner surfaceof the chamber is surface-treated in the following way: The surface issubjected to combined electrolytic polishing and electrolytic polishingand thereafter mirror-finished to a smoothness of maximum heightroughness Rmax<0.1 μm, on the surface of which a passive oxide film isfurther formed by the use of high-purity oxygen. The material is by nomeans limited to the high-purity oxygen so long as any impuritycontamination hardly occurs when the substrate temperature is elevated.

A gas exhaust system is comprised of the tandem type turbo molecularpump 25 comprising magnetic levitation type turbo molecular pumpsarranged in tandem and the dry pump 26 serving as an auxiliary pump.This exhaust system is an oil-free system that causes only slightimpurity contamination. The second-stage turbo molecular pump is a pumpof a large flow rate type, and can maintain exhaust velocity even at adegree of vacuum on the order of 10⁻³ Torr during plasma generation. Thegas exhaust system, including a mass flow controller and filters, is setup using SUS316 as in the case of the vacuum chamber and its innersurface is also subjected to surface treatment by polishing andformation of a passive oxide film so that any unauthorized impuritiescan be prevented from entering when processing gases are fed into it.

The substrate 16 is guided into the vacuum chamber 11 through a loadlock chamber (not shown) provided adjoining the vacuum chamber, and thusthe unauthorized impurities are prevented from entering in the vacuumchamber.

As a gas feed system, an ultraclean gas feed system and/or an oil-freeultrahigh-vacuum extraction system is/are employed so that the plasmaconstituent atoms other than the film constituent atoms can be preventedfrom being included in the film during the formation of epitaxial films.

A high-frequency energy may preferably be applied to the substrate inorder to form a more stable plasma. Especially when the substrate iscomprised of an insulating material, the frequency of high-frequencyenergy may be adjusted, so that the potential of the substrate surfacecan be adjusted. The high-frequency energy applied to the substrate maypreferably have a frequency higher than that of the high-frequencyenergy applied to the target.

As the processing gas to generate plasma, a high-purity gas comprisinghydrogen gas mixed with a single material inert gas such as argon orhelium or a mixed gas of several kinds of these inert gases. Here, theH₂ O gas, CO gas and CO₂ gas contained in the processing gas may eachpreferably be in a content of 1 ppm or less, and more preferably 100 ppbor less.

The pressure inside the vacuum chamber in the course of the filmformation may be kept within the range where discharge takes place. Inparticular, it may preferably be within the range of from 1 to 50 mTorr.

The substrate may preferably have a clean, bare surface. For example,the substrate provided in the vacuum chamber may be subjected to surfacecleaning by irradiation with ions while controlling high-frequencypower, DC voltage, gas pressure and so forth so that its surface mayhave no damage due to irradiation with ions. Here, for the same reasonsas in the film formation, the substrate temperature should be in therange of from 400° C. to 700° C., and the cleaning gas should have ahigh purity, comprising hydrogen gas mixed with a single material inertgas such as argon or helium or a mixed gas of two or more kinds of inertgases. H₂ O gas, CO gas and CO₂ gas should each preferably be made tohave a partial pressure of 1.0×10⁻⁸ Torr or less.

In the film formation, the gas pressure inside the chamber, thehigh-frequency power supplied to the target, the DC voltage applied tothe target and the high-frequency power optionally applied to thesubstrate may preferably be controlled so that the energy possessed byone ion made incident on the substrate can be controlled within therange that may cause no damage on the substrate. When, for example, thesubstrate is formed of Si and the plasma is mainly comprised of argon,the energy possessed by one argon ion should be controlled to be 40 eVor less. The value 40 eV corresponds to the threshold value at which thesputtering of Si is caused by argon, and is an upper limit of ionirradiation energy that causes no damage on the substrate surfacecomprising Si.

As a substrate heating means, a lamp irradiation means such as a xenonlamp or a halogen lamp that causes fewer impurities or a resistanceheating means should be used.

In the method of the present invention, a monocrystalline film of thesubstance constituting the target is epitaxially grown in accordancewith the information obtained on crystal orientation of themonocrystalline substrate used as a base. At this time, the dopant inthe target is incorporated into the film formed, in the same proportionas it is contained in the target. For example, when a Si film is formedusing a Si target containing the dopant in a given proportion, theresistivity as properties of the resulting film can be optimized to anextent substantially equal to that of bulk Si. Especially when thesubstrate temperature is within the range of from 500° C. to 600° C.,the resistivity of the film formed is 100 to 110% of that of bulk Si,showing very good properties. A film formed at substrate temperaturesranging from 500° C. to 550° C. has the smallest resistivity. This factshows that almost all the dopant in the film has been activated.

According to the method of the present invention, carbon atoms includedinto the film can be greatly decreased on account of their chemicalreaction with hydrogen, so that high-quality epitaxial films can beformed. According to the method of the present invention, the substratetemperature is kept not lower than 400° C., and hence the hydrogenhaving the function of removing unauthorized impurities can be veryeasily prevented from being included into the epitaxial film. If thesubstrate temperature is lower than 400° C., the incorporation ofhydrogen into the film may increase to greatly deteriorate film quality.If, on the other hand, the substrate temperature is higher than 700° C.,the partial pressure of CO and CO₂ around H₂ O may increase todeteriorate film quality and also to make it impossible to meet thedemand for making dopant profiles sharper or shallower.

EXAMPLES

The present invention will be described in greater detail by givingExamples. The present invention is by no means limited by these.

Example 1

Using the bifrequency excitation type bias sputtering apparatus shown inFIG. 1, epitaxial films were formed on Si (100) just-oriented substratesin the following way. As the target, p-type FZ 120 mm. diameter Si dopedwith boron to have a resistivity of 0.014 Ω·cm was used.

First, as shown in FIG. 2A, thermal oxidation was carried out on the Sisubstrate 42 at 1,000° C. for 54 minutes by resistance heating in anenvironment comprising H₂ gas and O₂ gas fed at a flow rate of 2 l/minand 4 l/min, respectively, to form an SiO₂ film 43 with a thickness ofabout 200 nm. From the upper part of this SiO₂ film 43, a resist 44 wasspin-coated, followed by known exposure and development. Thereafter, theresulting substrate was moved to a parallel flat type RIE (reactive ionetching) apparatus, and ion etching was carried out under the followingconditions to remove the SiO₂ film so that a pattern was formed on theSi substrate as shown in FIG. 2B. This pattern was comprised of aplurality of valleys of 1 μm wide and 1 μm deep and a plurality ofsquares of 10 μm×10 μm to 1 mm×1 mm in size as a plannar shape viewedfrom the upper part.

                  TABLE 1                                                         ______________________________________                                        RIE conditions                                                                ______________________________________                                        Gas flow rate: CHF.sub.3, 30 sccm                                                                          O.sub.2, 5 sccm                                  RF powder:     700 W                                                          Pressure:      0.13 Torr                                                      Time:          3 minutes                                                      ______________________________________                                    

From the substrate having been subjected to patterning in this way, theresist was separated to obtain the substrate as shown in FIG. 2C.

Next, this substrate was cleaned by a known wet process, and then set onan electrostatic chuck serving as the substrate holding member 14provided in the chamber 11 of the bias sputtering apparatus shown inFIG. 1, followed by heating to have a substrate temperature of 550° C.Subsequently, argon gas and hydrogen gas were fed in a given proportionfrom a gas feed system comprising an ultragas feed clean system, and thepressure inside the vacuum chamber was adjusted to 15 mTorr. DC voltageon the side of the substrate 16 and DC voltage on the side of the target12 were set at prescribed values as shown below and a high-frequencypower was supplied to the chamber 11 to thereby carry out ionirradiation surface cleaning by the action of plasma to remove moleculesadsorbed on the substrate surface. Next, the DC voltage on the side ofthe substrate 16, the DC voltage on the side of the target 12 and thehigh-frequency power were immediately set at prescribed values as shownbelow, and an Si epitaxial film with a thickness of about 100 nm wasformed at a substrate temperature of 550° C. Cleaning of the substrateand the film formation were carried out under conditions also shownbelow, provided that the frequency of the high-frequency power suppliedfrom the target side was kept constant at 100 MHz.

Substrate Surface Cleaning Conditions

Target side high-frequency power: 5 W

Target side DC voltage: -5 V

Ar gas pressure: 15 mTorr

H₂ gas/(Ar+H₂) gas volume ratio: 20%

Substrate side DC voltage: 10 V

Substrate temperature: 550° C.

Processing time: 5 minutes

Si Epitaxial Film Forming Conditions

Target side high-frequency power: 100 W

Target side DC voltage: -150 V

Ar gas pressure: 15 mTorr

H₂ gas/(Ar+H₂) gas volume ratio: 10 to 50%

Substrate side DC voltage: 10 V

Substrate temperature: 550° C.

Processing time: 5 minutes

Plasma potential (hereinafter "Vp") at the time of the surface cleaningand at the time of the Si film formation was measured by a conventionalLangmuir probe method to obtain the results as shown in FIG. 3. In thedrawing, the Vp is indicated as the ordinate and the hydrogen gascontent in the mixed gas of argon gas and hydrogen gas is indicated asthe abscissa in volume ratio. In the present Example, the Vp increasedwith an increase in the content of hydrogen gas in the mixed gas. Atthis time, substrate potential (hereinafter "Vs") is fixed and keptconstant by the DC power source on the substrate side. The ionirradiation energy of the ions made incident on the substrate surface isequal to the potential difference between the Vp and the substratepotential Vs, and hence, when the Vp varies, the ion irradiation energycan be controlled by changing the substrate potential Vs. Thus, thesubstrate potential Vs was also changed as the plasma potential Vpvaried.

With regard to thin films formed on substrates in the manner asdescribed above;

(1) crystal analysis was carried out by electron ray diffraction;

(2) content of impurity atoms of argon, carbon and hydrogen each wasmeasured by SIMS (secondary ion mass spectroscopy);

(3) reverse-current density at p-n junctions at the patterned portionswas measured; and

(4) step coverage was evaluated by SEM (scanning electron microscopy)observation.

As a result of the electron ray diffraction, it was ascertained that allthe thin films were epitaxially grown without any great difference.

Concentration of carbon atoms incorporated into the film was graphicallyrepresented, as shown in FIG. 4. In the drawing, carbon atomconcentration in the epitaxial film was indicated as the ordinate, andthe hydrogen gas content in the mixed gas of argon gas and hydrogen gasis indicated as the abscissa in volume ratio. As shown in FIG. 4, thecarbon atom concentration in the film decreases with an increase in thecontent of hydrogen gas in the mixed gas, and it can be ascertained thatthe hydrogen gas is effective for preventing inclusion of carbon atoms.

FIG. 5 shows the relationship between the reverse-current density at p-njunctions and the hydrogen gas content. In the drawing, thereverse-current density is indicated as the ordinate, and the hydrogengas content in the mixed gas of argon gas and hydrogen gas is indicatedas the abscissa in volume ratio. As shown in FIG. 5, in the presentExample, a minimum value is seen in the vicinity of 30 vol. % for thehydrogen gas content. This is considered due to the fact thatrecombination currents have been suppressed because the carbonconcentration in the film decreases so far as the hydrogen content is 30vol. % As for the step coverage, no great difference ascribable tohydrogen gas was seen.

Example 2

Next, epitaxial films were formed in the same manner as in Example 1except that the H₂ /(Ar+H₂) ratio was fixed at 20%, while changing thesubstrate temperature in the range of from 300° C. to 800° C. Propertiesof epitaxial films thus obtained are shown in Table 2.

                                      TABLE 2                                     __________________________________________________________________________    Substrate temperature                                                         300° C. 400° C.                                                                       500° C.                                                                        550° C.                                                                        600° C.                                                                        700° C.                                                                        800°             __________________________________________________________________________                                                          C.                      Resistivity (Ω · cm):                                          1.9 × 10.sup.-2 C                                                                      1.5 × 10.sup.-2 A                                                              1.4 × 10.sup.-2 A                                                               1.4 × 10.sup.-2 A                                                               1.5 × 10.sup.-2                                                                 1.6 × 10.sup.-2                                                                 9.0 ×                                                                   10.sup.-2 D             Argon content (cm.sup.-3):                                                    8.0 × 10.sup.18 D                                                                      1.3 × 10.sup.18 B                                                              1.0 × 10.sup.18 B                                                               8.8 × 10.sup.17 A                                                               8.0 × 10.sup.17                                                                 6.0 × 10.sup.17                                                                 8.0 ×                                                                   10.sup.18 D*            Carbon content (cm.sup.-3):                                                   6.0 × 10.sup.16 A                                                                      8.8 × 10.sup.16 A                                                              1.5 × 10.sup.17 A                                                               2.1 × 10.sup.17 B                                                               3.1 × 10.sup.17                                                                 6.0 × 10.sup.17                                                                 1.4 ×                                                                   10.sup.19 D             Hydrogen content (cm.sup.-3 ):                                                1.3 × 10.sup.20 D                                                                      9.4 × 10.sup.18 B                                                              1.1 × 10.sup.18 B                                                               4.3 × 10.sup.17 A                                                               9.7 × 10.sup.16                                                                 9.7 × 10.sup.16                                                                 9.7 ×                                                                   10.sup.16 A             p-n Junction reverse-                                                         current density (A/cm.sup.2):                                                 1.9 × 10.sup.-9 C                                                                      5.5 × 10.sup.-10                                                               2.0 × 10.sup.-10 A                                                              2.0 × 10.sup.-10 A                                                              4.0 × 10.sup.-10                                                                6.0 × 10.sup.-10                                                                1.4 ×                                                                   10.sup.-8 D             Electron ray diffraction:                                                     Streaks B      K-lines A                                                                            K-lines A                                                                             K-lines A                                                                             K-lines A                                                                             K-lines A                                                                             Streaks C               Step coverage:                                                                C              B      A       A       A       A       A                       Background vacuum degree (Torr):                                              2.0 × 10.sup.-10                                                                       4.0 × 10.sup.-10                                                               8.0 × 10.sup.-10                                                                3.0 × 10.sup.-9                                                                 7.0 × 10.sup.-9                                                                 1.0 × 10.sup.-8                                                                 8.0 ×                                                                   10.sup.-8               __________________________________________________________________________     K-lines: Kikuchi lines, A: Very good, B: Good, C: Ordinary, D: Poor, --:      Unmeasuarable *Crystallizability was poor and after cause an increase in      the Ar content.                                                          

As shown in Table 2, a large quantity of hydrogen atoms remain when thesubstrate temperature is lower than 400° C. On the other hand, when itis higher than 700° C., the argon content and carbon content increaseand also the reverse-current density at p-n junctions increases. Thedopant profile began to become a little dull. The argon content is smallwhen the substrate temperature is in the range of from 400° C. to 700°C. With regard to the reverse-current density at a p-n junction reversevoltage of 5 V, it is ascertained that minimum values are seen when thesubstrate temperature is 500 to 550° C. and the value abruptly increasesthe substrate temperature is lower than 400° C. or higher than 700° C.The step coverage is greatly improved when it is 400° C. or above.

In an instance in which the substrate temperature and the H₂ /(Ar+H₂)ratio were set constant at 550° C. and 20%, respectively, without anychanges in other conditions but the partial pressure of H₂ O, CO and CO₂each was made higher than 1×10⁻⁸, electron ray diffracted images becamegreatly irregular to decrease crystallizability.

Example 3

A bipolar transistor (BPT) was fabricated in the following way.

FIG. 6 illustrates an example of the constitution of a base region inthe BPT.

On the surface of a p-type Si substrate 31 with a resistivity of 4 Ω·cm,an n⁺ -type buried layer 32 was formed by a conventional diffusionprocess.

On the buried layer 32 thus formed, an n⁻ -type region 33 with a layerthickness of 1.2 μm was formed by a conventional epitaxial film formingprocess.

Subsequently, an n⁺ -type region 34 for decreasing the collectorresistance of the BPT was formed in the n⁻ -type region 33 by aconventional diffusion process.

Next, in the n⁻ -type region 33, As ions were implanted at a dose of2×10¹³ cm⁻² by conventional ion implantation to form a channel stopregion 36, followed by normal pressure CVD to form an SiO₂ film, whichwas then etched according to the RIE etching step previously described,to form a device separating region 35. Reference numeral 37 denotes a p⁺-type base region formed of a film epitaxially grown by bias sputtering.

Next, using the RF-DC combined bias sputtering apparatus as shown inFIG. 1, an about 300 nm thick Si epitaxial film was formed as a baseregion by the use of a target having a boron concentration of 1.0×10¹⁸cm⁻³. The film was formed under conditions shown below.

Substrate Surface Cleaning Conditions

Target side high-frequency power: 5 W

Target side DC voltage: -5 V

Ar gas pressure: 15 mTorr

H₂ gas/(Ar+H₂) gas volume ratio: 20%

Substrate side DC voltage: 10 V

Substrate temperature: 550° C.

Processing time: 5 minutes

Si Epitaxial Film Forming Conditions

Target side high-frequency power: 100 W

Target side DC voltage: -150 V

Ar gas pressure: 15 mTorr

H₂ gas/(Ar+H₂) gas volume ratio: 10%

Substrate side DC voltage: 5 V

Substrate temperature: 550° C.

Processing time: 15 minutes

The Vp in the course of the Si film formation was 25 eV. The Vs was setat 5 eV. Hence, the argon ions in plasma gained an energy of 20 eVcorresponding to the potential difference between them, with which thesubstrate had been irradiated. This value was a value suited to carryout epitaxial growth without damaging the substrate. Meanwhile, the Vpand Vs in the course of the substrate surface cleaning was 18 eV and 10eV, respectively, and the argon ion irradiation energy on the substratesurface was 8 eV.

Then, a substrate with contact holes made therein for forming an emitterwas made ready, and was cleaned by a conventional wet process, followedby RF-DC combined bias sputtering again, to form an about 500 nm thickSi epitaxial film on the substrate. The target was comprised of 1×10²¹cm⁻³ n⁺ FZ (100) Si. The film as formed under conditions shown below.

Substrate Surface Cleaning Conditions

Target side high-frequency power: 5 W

Target side DC voltage: -5 V

Ar gas pressure: 15 mTorr

H₂ gas/(Ar+H₂) gas volume ratio:. 20%

Substrate side DC voltage: 15 V

Substrate temperature: 550° C.

Processing time: 5 minutes

Si epitaxial film forming conditions

Target side high-frequency power: 100 W

Target side DC voltage: -400 V

Ar gas pressure: 15 mTorr

H₂ gas/(Ar+H₂) gas volume ratio: 0%

Substrate side DC voltage: 5 V

Substrate temperature: 550° C.

Processing time: 5 minutes

The Vp in the course of the Si film formation decreased to 20 eV sincethe target potential was set at -400 eV. The Vs was set at 0 eV. Hence,the argon ions in plasma gained an energy of 20 eV corresponding to thepotential difference between them, with which the substrate had beenirradiated. This value was a value suited to carry out epitaxial growthwithout damaging the substrate. Meanwhile, the Vp and Vs in the courseof the substrate surface cleaning was 18 eV and 10 eV, respectively, andthe argon ion irradiation energy on the substrate surface was 8 eV.

The resulting film was subjected to patterning by a conventional processto form an emitter region. FIG. 7 illustrates an example of theconstitution of the emitter region in the BPT.

An insulating film 39 comprised of SiO₂ with a layer thickness of 0.5 μmwas deposited by conventional normal pressure CVD, and contact holeswere formed by patterning, followed by conventional sputtering tosuperpose TiN and Al--Si--Cu in layers to form wiring electrodes 40.

An insulating film 41 comprised of SiO₂ was deposited, and an externalwithdrawing outlet was worked by a conventional semiconductor process.

The foregoing schematically illustrates a process for fabricating theBPT. What are most important in the process are the steps of forming thebase and the emitter that influence the performance of the BPT. First,in the base region, in order to obtain a high-performance BPT, it isstrongly demanded to make base resistance lower, suppress recombinationcurrents in the base and at the base interface, and decreaseirregularities thereof. Hence, it is strongly demanded to make the baseregion shallower, make the dopant profile more uniform, and make thebase-collector interface and base-emitter interface clean and flat. Thepresent invention has made it possible to achieve the film quality andinterfacial state that can meet such demands. That is, the presentinvention has made it possible to obtain an epitaxial interface having alow reverse-current density and a sharp dopant profile and also freefrom unauthorized impurities such as naturally occurring oxide films.The present bias sputtering also enables easy formation of thin films,and is advantageous for making the profile shallower. It has beenascertained that BPTs having base regions of up to 10 nm can befabricated and operated.

FIG. 8 is a graph to show the dependence of film formation rate ontarget bias in the bias sputtering. Film forming conditions other thantarget bias are made constant, which are as shown below.

Substrate Surface Cleaning Conditions

Target side high-frequency power: 5 W

Target side DC voltage: -5 V

Ar gas pressure: 15 mTorr

H₂ gas/(Ar+H₂) gas volume ratio: 20%

Substrate side DC voltage: 10 V

Substrate temperature: 550° C.

Processing time: 5 minutes

Si Epitaxial Film Forming Conditions

Target side high-frequency power: 100 W

Target side DC voltage: --50 to --400 V

Ar gas pressure: 15 mTorr

H₂ gas/(Ar+H₂) gas volume ratio: 10%

Substrate side DC voltage: 5 V

Substrate temperature: 550° C.

For example, when the target bias was --50 V, a film 10 nm thick wasobtained in about 10 minutes and the film showed a goodcrystallizability.

As for the emitter region, the recombination and irregularities at theemitter-base interface are as described above. With regard to theachievement of a lower emitter resistance, the emitter is comprised ofmonocrystalline Si while conventional DOPOS (doped poly-Si) emitters arecomprised of polycrystalline Si, and hence it has become possible toachieve a lower emitter resistance.

Example 4

In the present Example, the apparatus in Example 1 previously describedwas used, and a p-type 1.0×10¹⁵ cm⁻³ boron-doped Si (111) FZ wafer (made4°-off in the direction of <211>) was used as the substrate. Substratesurface cleaning and film formation were carried out under conditionsshown below.

Substrate Surface Cleaning Conditions

Target side high-frequency power: 5 W

Target side DC voltage: -5 V

Xe gas pressure: 15 mTorr

H₂ gas/(Xe+H₂) gas volume ratio: 20%

Substrate side DC voltage: 9 V

Substrate temperature: 550° C.

Processing time: 5 minutes

Si Epitaxial Film Forming Conditions

Target side high-frequency power: 200 W

Target side DC voltage: -200 V

Xe gas pressure: 15 mTorr

H₂ gas/(Xe+H₂) gas volume ratio: 20%

Substrate side DC voltage: -7 V

Substrate temperature: 550° C.

Processing time: 5 minutes (150 nm)

The Vp in the course of the Si film formation was 22 eV. The Vs was setat -7 eV. Hence, the xenon ions in plasma gained an energy of 29 eVcorresponding to the potential difference between them, with which thesubstrate had been irradiated. This value was a value suited to carryout epitaxial growth without damaging the substrate. Meanwhile, the Vpand Vs in the course of the substrate surface cleaning was 17 eV and 9eV, respectively, and the xenon ion irradiation energy on the substratesurface was 8 eV.

Optimum values of plasma parameters at the time of Si film formation aredifferent from those in the case of the Si (100) substrate as shown inExample 1 because of, e.g., the difference in surface energy between Si(111) and Si (100). However, with regard to the properties of Si thinfilms and others, very good films showing substantially the same resultswere obtained.

Example 5

In the present Example, the apparatus in Example 1 previously describedwas used, and Si (001) monocrystalline film was formed on the (1012)plane of an insulating material monocrystal sapphire (Al₂ O₃) substrate.Since the substrate is an insulating material, the energy of ions withwhich the substrate is irradiated, influenced by the potential of thesubstrate surface (i.e., the difference between plasma potential andsubstrate potential), depends on the high-frequency power supplied tothe substrate and the frequency of the high-frequency power. As thetarget material, an n-type 1.8×10¹⁸ cm⁻³ phosphorus-doped FZ Si (111)substrate was used, and Si was heteroepitaxially grown. Substratesurface cleaning and film formation were carried out under conditionsshown below.

Substrate Surface Cleaning Conditions

Target side high-frequency power: 5 W

Frequency of target side high-frequency power: 100 MHz

Target side DC voltage: -5 V

Substrate side high-frequency power: 10 W

Frequency of substrate side high-frequency power: 190 MHz

Ar gas pressure: 15 mTorr

H₂ gas/(Ar+H₂) gas volume ratio: 20%

Substrate side DC voltage: 5 V

Substrate temperature: 550° C.

Processing time: 5 minutes

Si Epitaxial Film Forming Conditions

Target side high-frequency power: 200 W

Frequency of target side high-frequency power: 100 MHz

Target side DC voltage: -200 V

He+Ar gas pressure: 15 mTorr

H₂ gas/(He+Ar+H₂) gas volume ratio: 20%

Substrate temperature: 550° C.

Processing time: 5 minutes (1,500 nm)

The Vp in the course of the Si film formation was 31 eV. The Vs was setat -1 eV. Hence, the argon ions in plasma gained an energy of 32 eVcorresponding to the potential difference between them, with which thesubstrate had been irradiated. This value was a value suited to carryout epitaxial growth without damaging the substrate. Meanwhile, the Vpand Vs in the course of the substrate surface cleaning was 22 eV and 8eV, respectively, and the argon ion irradiation energy on the substratesurface was 14 eV.

The thin film thus formed was subjected to light etching, a processcarried out to make any crystal defects conspicuous, to observe etch pitdensity and also subjected to sectional TEM (transmission electronmicroscopy) to observe defect density such as layer defects or transfer.As a result, both the etch pit density and the defect density were each1.0×10⁷ to 1.0×10⁸ pits or defects/cm.

In order to examine inclusion of Al into the Si thin film, measurementby SIMS was carried out to reveal that Al concentration in the Si thinfilm was a measurement limit 2×10¹⁵ cm⁻³ or less. Hole mobility ofelectrons was also measured by hole measurement to reveal that it showeda value of 240 cm² /V·sec at normal temperature of 300 K, which wassubstantially equal to that of the bulk.

As is clear from the foregoing results, an SOS (sapphire on silicon)thin film was obtained. With regard to the problem of elasticcompression strain occurring in the Si thin film because of a differencein the coefficient of thermal expansion between sapphire and Si(9.5×10⁻⁶ /° C. and 4.2×10⁻⁶ /° C., respectively), it was 1×10⁸ dyne/cm²or less because the growth temperature as low as 550° C., brings about agreat improvement.

As is clear from what has been described above, the method for formingan Si-deposited film, of the present invention makes it possible tostably form high-quality monocrystalline thin films having a good stepcoverage and causing less film deposition defects. Since the films canbe formed at low temperatures, it is also possible to prevent the dopantin the film from diffusing into unauthorized regions of the filmdeposited regions and to form a sharp profile.

Example 6

Example 6 is an example in which poly-Si (polycrystalline Si) films areformed on Si (100) substrates, using the bifrequency excitation typebias sputtering apparatus shown in FIG. 1. As the target, p-type FZ120mm diameter Si doped with boron to have a resistivity of 1.5 Ω·cm wasused. To form the oxide film, thermal oxidation was carried out on theSi substrate at 1,000° C. for 54 minutes by resistance heating in anenvironment of H₂ gas:O₂ gas=2 l/min:4 l/min.

Next, the resulting substrate was cleaned by a known wet process, andthen set on an electrostatic chuck serving as the substrate holdingmember 14 provided in the chamber 11 of the bias sputtering apparatusshown in FIG. 1, followed by heating to have a substrate temperature of550° C. Subsequently, argon gas and hydrogen gas were fed in a givenproportion from a gas feed system comprising an ultragas feed cleansystem, and the pressure inside the vacuum chamber was adjusted to 15mTorr. DC voltage on the side of the substrate 16 and DC voltage on theside of the target 12 were set at prescribed values as shown below and ahigh-frequency power was supplied to the chamber 11 to thereby carry oution irradiation surface cleaning by the action of plasma to removemolecules adsorbed on the substrate surface. Next, the DC voltage on theside of the substrate 16, the DC voltage on the side of the target 12and the high-frequency power were immediately set at prescribed valuesas shown below, and a poly-Si film with a thickness of about 100 nm wasformed at a substrate temperature of 550° C. Cleaning of the substrateand the film formation were carried out under conditions also shownbelow, provided that the frequency of the high-frequency power suppliedfrom the target side was kept constant at 100 MHz.

Substrate Surface Cleaning Conditions

Target side high-frequency power: 5 W

Target side DC voltage: -5 V

Ar gas pressure: 15 mTorr

H₂ gas/(Ar+H₂) gas volume ratio: 20%

Substrate side rf frequency: 190 MHz

Substrate side high frequency power: 5 W

Substrate temperature: 550° C.

Processing time: 5 minutes

Poly-Si Film Forming Conditions

Target side high-frequency power: 100 W

Target side DC voltage: -150 V

Ar gas pressure: 15 mTorr

H2 gas/(Ar+H₂) gas volume ratio: 10 to 50%

Substrate side high frequency: 190 MHz

Substrate side high frequency power: 20 W

Substrate temperature: 550° C.

Processing time: 10 minutes

Plasma potential at the time of the surface cleaning and at the time ofthe poly-Si film formation was measured by a conventional Langmuir probemethod. As a result, the energy of ions applied on the substrate was 8eV at the time of the surface cleaning and 30 eV at the time of thepoly-Si deposition. The deposited film was in a thickness of about 100nm, and a poly-Si film oriented to (111) was formed. Its average crystalgrain diameter was 800 angstroms and surface roughness was 10 nm as apeak-to-valley value (PV value). Regarding the orientation, theorientation to the (111) closest packed face can be made easily bydepositing the film while ion irradiation is carried out. While theconditions depend on ionic energy, substrate temperature and so forth,the orientation has a strong dependence on the energy of ions applied onthe surface, and can be most easily made at 25 eV to 40 eV as ionirradiation energy. With regard to the surface roughness, smoothness isimproved when the product of ionic energy value and irradiation densityis great. If, however, the ionic energy value is too large, the damagedue to sputtering becomes greater to cause an increase in surfaceroughness. It is most effective to increase the irradiation density, andaccordingly the ionic energy value is made smaller and the energyimparted only to Si atoms present in the vicinity of the surface is madegreater, whereby the surface migration can be promoted and the surfaceenergy can become smaller, i.e., the surface smoothness can improve.Raising substrate temperature may also be effective. In the presentExample, the carbon atom content, hydrogen atom content and Ar atomcontent were 9×10¹⁷ cm⁻³, 9×10¹⁸ cm⁻³ and 1×10¹⁹ cm⁻³ , respectively.

We have mentioned forming the polycrystalline Si so far, if thesubstrate temperature is reduced to 400° C., we also can form theamorphous Si with good quality. Therefore, we can attain moretemperature reduction of the substrate.

A thin-film transistor (TFT) was further formed on this poly-Si film.First, the poly-Si film formed in the manner described above wassubjected to patterning, and a gate oxide film was formed thereon.Taking account of substrates (e.g., glass substrates) that must beprocessed at low temperatures, the gate oxide film was formed as adeposited film by CVD (chemical vapor deposition), though resulting insome lowering of characteristics. A thermal oxide film, having lessboundary levels, may be used as the gate oxide film, but on the otherhand there are disadvantages such that high-temperature processing isrequired, substrate materials are restricted, and hydrogen escapes atthis step. No particular limitations are required. TFT of a stagger typewill be described here. Needless to say, a structure of a reversestagger type in which the gate oxide film is used as a base of thepoly-Si film may also be used without any problem at all. As a nextstep, a poly-Si film as a gate electrode is deposited. The Si filmhaving been described in the present invention may be used, or the filmmay be formed by LPCVD (low-pressure chemical vapor deposition) usuallyused. In the case when the Si film described in the present invention isused, any films containing a dopant as the target at a high density maybe used, so that it becomes possible to omit the step of ionimplantation carried out in a subsequent step. After the poly-Sielectrode has been subjected to patterning, source-drain ionimplantation is carried out. If necessary, an LDD structure or offsetstructure may be used. In the present Example, a 1 μm offset structurewas used. Subsequently, the oxide film, an interlayer film, was formedand then subjected to patterning. Thereafter, an Al/Si electrode asformed and then subjected to patterning. Since in the present inventionthe hydrogen has been already incorporated into the Si film at the timeof poly-Si deposition, the step of hydrogenation by plasma-assisted CVDis not taken. The TFT thus formed had very good characteristics, havinga mobility of 50 cm² /Vsec and an off-leak of 3×10⁻¹² /μm. ItsS-coefficient was 212 mV/decade, and also gate oxide film breakdownstrength was 8 MV/cm or more, which were good characteristics for TFToperation.

Thus, the poly-Si can be deposited without use of complicated processessuch as laser annealing and solid-phase growth, the process can besimplified and the TFT with good characteristics can be formed at alow-temperature process. Hence, this brings about a very great effectthat the films can be formed inexpensively at a high degree of freedomon substrates. This TFT also has a great effect on channel areas and isvery effective in view of characteristics, without particularity aboutthe stagger type or the reverse stagger type of course. Source and drainregions may also be formed by in situ doping, using the Si layer of thepresent invention, whereby the step of ion implantation can besimplified. In combination with the poly-Si electrode described above,the TFT can also be formed without the process of ion implantation atall, promising a high degree of freedom. These steps may also be carriedout in the same chamber, whereby many processes can be performed withoutbreak of vacuum, and TFT with much better characteristics can beobtained.

Example 7

Example 7 is an example in which the TFT described in Example 6 isemployed in a liquid-crystal display device.

FIG. 9 illustrates a drive circuit of an active matrix typeliquid-crystal display device.

The drive circuit shown in FIG. 9 is constituted of a picture elementarea comprised of a liquid-crystal display cell 1401 having aliquid-crystal material sealed between a common electrode (potential:represented as V_(COM)) and each picture element electrode, an imagesignal wiring area (hereinafter "signal wiring") 1403, a line bufferarea 1404, a shift pulse switch 1408, an output switch 1410, ahorizontal shift register 1405, a gate signal wiring area (hereinafter"gate wiring") and a vertical shift register 1406. Recording signals aretransmitted, while displacing the timing, from a signal input terminal1407 successively to each picture element or each line.

FIGS. 10A and 10B illustrate drive pulse timing of this conventionalactive matrix liquid-crystal display device. In these figures, a linesequential driving method is illustrated. More specifically, imagesignals to be recorded in the crystal liquid pass through the shiftpulse switch 1408 which is driven by the horizontal shift register 1405synchronized with the frequency of the image signals, and image signalsfor one line are recorded in the line buffer area 1404 (FIG. 10A). Afterimage signals of all picture elements for a certain line have beenrecorded in the line buffer area 1404, the image signals are recorded ineach liquid-crystal cell through the picture element switch turned on byoperating the output switch 1410 of the line buffer area 1404 and thevertical shift register 1406. Usually the signals are transmitted toeach liquid-crystal cell in bundle for a certain line during a blankinginterval in the period of horizontal scanning. According to the timingas stated above, the image signals are successively transmitted to eachline (FIG. 10B).

Liquid-crystal molecules constituting the cells act in response to thevoltage of the signals thus transmitted to cause changes in transmissionof the liquid-crystal cells in accordance with the direction of apolarizing plate separately provided in relation to a cross polarizer.This difference in transmission is represented as light and shade ofeach picture element, and thus the liquid-crystal cells operate as adisplay device.

In this switching transistor 1402, the TFT semiconductor device of thepresent invention was used. Since the TFT having a drive power, causingless leak currents in the off state and also showing good risecharacteristics can be readily formed by a low-temperature process, theactive matrix type liquid-crystal display device shown in FIG. 9 was setup inexpensively with good characteristics. The semiconductor device ofthe present invention may also be used in the horizontal shift registerand the vertical shift register, and also other process, e.g., laserannealing may be used when a higher performance is required. Noparticular limitations are required.

Example 8

Example 8 is an example of a liquid-crystal display in which TFT is madefrom poly-Si and peripheral driving circuit is made from bulk-Si, andExample 8 shows the manufacturing method of the same.

FIG. 11 illustrates the cross-sectional structure of an active matrixliquid-crystal display device and shows the features of Example 8.

In FIG. 11, reference numeral 401 denotes a gate electrode of athin-film transistor. Reference numeral 402 denotes a semiconductorlayer which is made of, for example, monocrystalline silicon,polycrystalline silicon or amorphous silicon to form a channel area ofthe thin-film transistor. Reference numeral 403 denotes a sourceelectrode. Reference numeral 404 denotes a drain electrode. Referencenumeral 405 denotes an interlayer insulator film. Reference numeral 406denotes a monocrystalline silicon substrate. Reference numeral 407denotes an orientation film. Reference numeral 408 denotes aliquid-crystal material. Reference numeral 409 denotes an opposedtransparent electrode. Reference numeral 410 denotes an interlayer film.Reference numeral 411 denotes a light blocking layer. Reference numeral412 denotes a color filter layer. In the case of a monochrome displaypanel, no color filter layer exists. Reference numeral 413 denotes anopposed transparent substrate. Reference numeral 414 denotes asemiconductor diffused layer formed in the monocrystalline siliconsubstrate to form a channel area of a transistor and having aconductivity type reverse from that of the monocrystalline siliconsubstrate.

Reference numeral 415 denotes a gate electrode which is formed in thisembodiment by the same process as that in which the gate electrode 401of the transistor is formed. However, the gate electrode 415 may beformed in another process. The gate electrode 415 and the channel area414 oppose each other with an insulator layer therebetween. Referencenumerals 416 and 417 are respectively source and drain electrodes of atransistor which are formed in the monocrystalline substrate. Referencenumeral 418 denotes an electrode of the semiconductor diffused area 414.Reference numeral 419 denotes a transparent pixel electrode connected tothe drain electrode 404. Reference numeral 420 denotes a storagecapacitor common electrode which forms a storage capacitor for holdingelectric charges of the pixel electrode portion. The common electrode420 and the pixel electrode 419 in combination form a storage capacitor.

In Example 8, a dielectric formed between the gate electrode 415 of themonocrystalline transistor formed in the monocrystalline siliconsubstrate and an inverted layer of the channel area 414 thereof is usedas a holding capacitor for a video signal. The dielectric is formed ofsilicon oxide. A video signal is held in the gate electrode 415 andrecorded in the pixel electrode at a desired timing. The pixel areaconsists of the thin-film transistor formed on the interlayer insulatorfilm 419 and the transparent pixel electrode 419 connected to the drainelectrode of that thin-film transistor. The portion of themonocrystalline silicon substrate, located below the pixel area, isremoved by etching, as shown in FIGS. 14A to 14C so that it can be seenthrough under visible light.

The holding capacitor is constituted using, as a dielectric layer, theinsulator film formed on the monocrystalline substrate. The pixel areaportion, which is constituted by the thin-film transistor on theinterlayer insulator film and the transparent electrode, has a structurein which the portion of the silicon substrate layer located below thepixel area portion is removed by etching so that it can be seen through.It is thus possible to provide a transmission type liquid crystaldisplay device having a highly reliable holding capacitor.

In Example 8, Si layer forming the channel area of the thin-filmtransistor is made by the method explained in Example 6. Therefore,off-leak current and light-leak current of this channel area are verysmall, and the thin-film transistor has very good quality. By using itthe liquid-crystal display with very high resolution can be realized.

FIGS. 12A and 12B, FIGS. 13A to 13C, FIGS. 14A to 14C, FIGS. 15A and15B, FIGS. 16A and 16B, FIGS. 17A to 17C, FIGS. 18A and 18B, and FIG. 19schematically illustrate the method of manufacturing the above-describedstructure.

FIG. 12A illustrates a step in which a p type well is formed in an ntype Si substrate 121. The p type well (PWL) is manufactured by maskingan oxide film 122 having a thickness of 200 Å formed on the substrateusing a resist 123 and by implanting boron ions. FIG. 12B illustrates astep in which a resist used for LOCOS (localized oxidation of silicon)is pasted. After the surface is oxidized, SiN (an oxidized nitridedfilm) is deposited on the surface and patterned.

FIG. 13A illustrates a step in which LOCOS is formed. A field oxide film131 having a thickness of 400 nm to 1500 nm is formed by conducting wetoxidation at a temperature ranging from 1000 to 1100 ° C. FIG. 13Billustrates a step in which SiN 133 is formed on the surface. A SiN filmis deposited to a thickness of 50 nm to 100 nm by thermal CVD, and abuffer oxide film 132 is deposited on the SiN film. FIG. 13C to FIG. 14Aillustrate steps in which polysilicon which forms pixel TFTs is formedon the surface. Polycrystalline Si is deposited to a thickness of 50 nmto 400 nm and patterned. Moreover, polycrystalline Si is deposited in athickness of 20 nm to 200 nm. This polycrystalline Si is produced in theabove Examples.

FIG. 14A illustrates a step in which p type ions are implanted to adjustthe threshold value of the n type TFT. Reference numeral 141 denotes TFTpoly-Si film. Ion implantation is conducted at a density of 10¹² to 10¹⁴cm⁻³. FIG. 14B illustrates a step in which polycrystalline Si depositedon the portion other than the upper surface of TFT is removed. FIG. 14Cillustrates a step in which a nitride film formed on the portion otherthan the upper surface of TFT is removed to expose the peripheralcircuit alone. Reference numeral 142 denotes SiN and reference numeral143 denotes gate oxide film.

FIG. 15A illustrates a step in which polycrystalline Si for the gateelectrode is formed. FIG. 15B illustrates a step in which the gateelectrode is patterned. Reference numeral 151 denotes poly-Si for gate.

FIG. 16A illustrate a step in which ion implantation is conducted on theTFT and peripheral circuit portion to form the source area and drainarea. FIG. 16B illustrates formation of the interlayer film.

FIG. 17A illustrates a step in which Al--Si is deposited to form anextension electrode. Reference numeral 171 denotes TiN and referencenumeral 172 denotes Al. FIG. 17B illustrates a step in which theinterlayer insulator film is formed. FIG. 17C illustrates a step inwhich surface protecting resist 173 is coated.

FIG. 18A illustrates a step in which TiN 181 for shielding the TFT fromlight is deposited on the upper surface of TFT. FIG. 18B illustrates astep in which the transparent pixel electrode is formed. Referencenumeral 182 denotes ITO.

FIG. 19 is a cross-sectional view of a liquid-crystal display devicecompleted by combining the thus-obtained substrate with an opposedsubstrate having the common electrode. Reference numeral 191 denotesorientation film; 192, liquid-crystal; and 193, filter substrate.

In the above examples, we have shown the transparent liquid-crystaldisplay. However, if the transparent electrode is replaced with themetal electrode and the Si substrate is not etched, it can be thereflective liquid-crystal display.

What is claimed is:
 1. A method for forming a crystalline Si film by abias sputtering process comprising the steps of:generating plasmabetween a target electrode holding a target material provided in avacuum container and a substrate electrode holding a substrate forforming thereon said crystalline Si film, provided opposingly to thetarget electrode, by the use of a high-frequency energy to cause thetarget material to undergo sputtering; and applying a bias voltage to atleast one of the target electrode and the substrate electrode to formsaid crystalline Si film comprised of atoms having been deposited bysputtering on the substrate, wherein; the target material beingsubjected to sputtering while controlling H₂ O gas, CO gas and CO₂ gasin the mixed-gas environment to have a partial pressure of 1.0×10⁻⁸ Torror less each, to form said crystalline Si film on the substrate whilemaintaining a substrate temperature in the range of from 400° C. to 700°C., wherein a mixed-gas environment comprising a mixture of a rare gasand a hydrogen gas which is in a content of 10% to 50% of the mixture isformed in said vacuum container, to form said crystalline Si film havinga carbon content of 5×10¹⁷ cm⁻³ or less, a hydrogen content of 1×10¹⁶cm⁻³ to 1×10¹⁹ cm⁻³ and a rare gas content of 1×10¹⁶ cm⁻³ to 1×10¹⁹cm⁻³.
 2. The method according to claim 1, wherein said crystalline Sifilm is monocrystalline Si.
 3. The method according to claim 1, whereinsaid crystalline Si film is polycrystalline Si.
 4. The method accordingto claim 1, wherein before said crystalline Si film is formed, thesubstrate is irradiated with ions in the plasma to clean the substrate.5. The method according to claim 4, wherein the frequency of thehigh-frequency voltage applied to the substrate is higher than thefrequency of the high-frequency voltage applied to the target.
 6. Themethod according to claim 1, wherein a DC voltage is applied to thesubstrate.
 7. The method according to claim 1, wherein a high-frequencyvoltage is applied to the substrate.
 8. The method according to claim 1,wherein the substrate temperature is within the range of 500° C. to 700°C.